The present invention relates to a device for monitoring a yarn reserve on a yarn spool from which the yarn is pulled off over a head.
Monitoring devices of the type under consideration include a feeler which is spring-biased and applied to a yarn package on the spool.
Textile machines which operate with yarn must be switched off when the yarn reserve on the spool is exhausted and the yarn end of this spool has not yet been connected with the yarn end of a new full spool. The spool holders with operating spools and reserve spools become obstructed and can not be employed at all places. Furthermore, the pull-off transition to the reserve spool with badly reeled spools would result in an additional source of disturbance.
However, to wait till the yarn would be completely pulled off the spool before the machine is switched off by a so-called thread watcher has been disadvantageous because it has been very difficult and troublesome to tie up the starting point of the yarn of the next yarn spool with the end of the yarn being pulled off the previous spool. With badly wound spools, there is the danger that in the last layers the remaining yarn turns would slide down and form a thread clew.
Devices have been proposed by means of which the start of pulling off the last yarn layer from the spool can be determined. With these conventional devices the end region of the body of the spool at the side opposite to that at which the yarn is pulled off, has been provided with at least one opening through which a feeler of the switching device has been applied to the innermost yarn layer; the feeler has performed a switching movement as soon as the yarn of the last thread layer on the spool has been removed at the place of sensing. These known devices, however have the disadvantage that specific yarn spools provided with openings for feelers have been required, and the yarn spool body must always cooperate with the switching device.
It has been also known to optically scan the yarn spools on their external sides and provide the surfaces to be wound upon with thread with a reflecting coating so that after removing the last wound yarn layer from the spool light reflection is obtained at the place of scanning, which can be utilized as a control signal for the switching off the textile machine. This monitoring device however has the disadvantage which resides in that this device can be utilized only in connection with specially prepared spools also provided with reflecting surfaces, and the application of such a device can cause sometimes false switchings of the machine due to light reflection when gloss yarns are used.